1. Field of the Invention
The present invention relates to the compensation of chromatic dispersion in a wavelength division multiplexing (WDM) optical transmission system. In particular, the present invention relates to compensation of the dependence of the chromatic dispersion on wavelength by means of a spectral inversion device.
The following definitions are used in the remainder of the description:
Optical channel: modulated signal on an optical carrier having a given central frequency. The modulated signal has a frequency much lower than the central frequency of the optical carrier. For the purposes of the present description, reference will be made to the corresponding central wavelength of the channel, rather than to its central frequency. The optical channel has a given spectral width; in other words, it comprises different spectral components.
Chromatic dispersion (or, more simply, dispersion): defined as D=d/dλ(1/vg), where vg is the group velocity of a spectral component at a given frequency of a pulse which is propagated along an optical path. Because of the chromatic dispersion, the different spectral components of an optical pulse travel along a dispersive optical path at slightly different velocities from each other, causing a temporal widening or narrowing of the pulse.
Accumulated chromatic dispersion along an optical path: for an optical path consisting of a plurality of optical path segments, each of length Li and dispersion D1(λ), this is defined as Dacc(λ)=ΣDi(λ)·Li. The summation is understood as an algebraic sum; in other words, the dispersions D1(λ) are taken with their sign. In practice, it corresponds to the chromatic dispersion measured at the output of the optical path for each wavelength.
Average chromatic dispersion of an optical path: for an optical path consisting of a plurality of optical path segments, each of length L1 and dispersion D1(λ), this is defined as Dave(λ)=ΣD1(λ)·L1/ΣL1. In other words, this is the accumulated chromatic dispersion on the optical path averaged over the whole length of the said optical path.
Spectral inversion: inversion of the spectral components of an optical channel with respect to its central frequency.
Optical phase conjugation: propagation of the phase conjugated of the electromagnetic field of an optical channel. The spectrum of an optical channel after a phase conjugation becomes a mirror image of the initial spectrum; in other words, the optical channel undergoes a spectral inversion.
The wavelength division multiplexing (WDM) technique can be used to transmit along a single optical fibre different channels modulated by signals at even high bit rates (for example, 10 Gbit/s or above), in such a way that the best use can be made of the optical transmission band provided by the optical fibre.
One of the problems of transmission along optical fibres is the compensation of chromatic dispersion, which causes a widening of a pulse transmitted along the optical fibre due to the different velocities of its spectral components. The result of the distortions introduced by chromatic dispersion is the degradation of the optical signal sent along the fibre, to levels which may be unacceptable.
2. Description of the Related Art
G. V. Agrawal, in Chapter 9 of his book “Fiber-optic Communication Systems”, Wiley-Interscience Publication of John Wiley & Sons, Inc., reviews different methods of dispersion compensation. These include a widespread method by which the optical system is formed by alternating optical fibres having opposite signs of chromatic dispersion, in such a way that the average dispersion of the whole optical path is zero. For example, this can be achieved by alternating standard single mode fibres, having a chromatic dispersion in the region of 1.55 μm, in the range from approximately 16 ps/nm·km and 20 ps/nm·km, with short segments of dispersion compensating fibres (DCF), having a strongly negative dispersion. These DCF fibres are normally located next to the optical amplifiers present along the transmission line. This method enables the dispersion of a single channel to be compensated completely.
When the WDM method is used, the whole set of transmitted channels can occupy a band of 20–30 nm or more. Since the chromatic dispersion is dependent on the wavelength, with a practically linear relationship having a generally non-zero slope in the transmission window around 1.5 μm, different channels accumulate different dispersion levels, and it therefore becomes more difficult to compensate the dispersion over the whole signal band. This problem is particularly important in high bit rate systems (>10 Gbit/s), for which the dispersion compensation has to be carried out very accurately. The dependence of chromatic dispersion on the wavelength is sometimes referred to in the literature as “second order dispersion”.
As far as broad-band dispersion compensation methods are concerned, in the case of a WDM system, G. V. Agrawal suggests, in Paragraph 9.8.2 of the cited book, the use of DCF fibres capable of compensating for dispersion throughout the signal band, such as DCF fibres with a negative dispersion slope, or the use of suitable filters, such as special optical fibre gratings (sampled fibre gratings).
According to the applicant, the use of DCF fibres which can simultaneously compensate the dispersion and the slope of the dispersion over the whole signal band is not an optimal solution, since these fibres, having very specific dispersion characteristics, are rather difficult to produce and therefore very expensive. Moreover, in previously installed systems this solution would require the replacement of all the DCF fibres of the system, with a considerable increase in cost.
The use of optical fibre gratings also has drawbacks, such as an imperfectly linear variation of dispersion with respect to wavelength, and a marked sensitivity to temperature variations, which makes it necessary to use suitable carefully designed packages in which the operating temperature is controlled.
As explained in Paragraph 9.7 of the previously cited book by G. V. Agrawal, another technique for compensating for chromatic dispersion introduced by an optical path is optical phase conjugation. This technique consists in propagating, from approximately halfway along the optical path, the phase conjugate of the electromagnetic field of an optical signal propagated along the path. From the mathematical point of view, this is equivalent to propagating the optical signal along a portion of optical path having a chromatic dispersion equal and opposite to that of the portion preceding the phase conjugator. Therefore, from the mathematical point of view, except for the phase inversion introduced by the phase conjugator, the “original” optical field (in other words that introduced at the input of the optical path) is completely restored, together with the original shape of the pulse. Since the spectrum of the signal is inverted after the phase conjugator, the phase conjugation method of dispersion compensation is also known as spectral inversion (“midspan spectral inversion”, or MSSI).
The practical implementation of the phase conjugation technique requires the use of a non-linear optical element to generate the phase conjugated signal. A commonly used method requires the use of four-wave mixing (FWM) in a non-linear medium. For example, the non-linear medium can be an optical fibre or a semiconductor amplifier. Further details can be found in the book by Agrawal cited above. In another example, I. Brener, M. H. Chou, E. Chaban, K. R. Parameswaran, M. M. Fejer, S. Kosinski and D. L. Pruitt, in “Polarization-insensitive wavelength converter based on cascaded nonlinearities in LiNbO3 waveguides”, Electronics Letters, vol. 36, no. 1, pp. 66–7 (2000), describe a wavelength conversion device based on periodically polarized lithium niobate integrated waveguides, which, according to these authors, can also be used as a spectral inverter.
The MSSI dispersion compensation technique has also been proposed and experimentally implemented for wavelength division multiplexing systems. For example, A. H. Gnauck, R. M. Jopson, P. P. Tannone and R. M. Derosier, “Transmission of two-wavelength-multiplexed 10 Gbit/s channels over 560 km of dispersive fibre”, Electronics Letters, vol. 30, no. 9, pp. 727–8 (1994), describe an experiment in which a phase conjugation technique is used to compensate the dispersion of two 10 Gbit/s channels, with wavelengths in the region of 1.5 μm, in 560 km of optical fibre having λ0 at 1.3 μm. The wavelengths of the two transmitted channels were 1542.2 nm and 1543.1 nm. The accumulated chromatic dispersion along the 560 km of the system was 9600 ps/nm. The phase conjugator was based on non-degenerate four-photon mixing (FPM) with pumping radiation polarized orthogonally with each other in a 7.8 km long dispersion shifted optical fibre (DSF). Two pumping radiations, at 1547.2 nm and 1548.2 nm respectively, were combined together by means of a polarization beam splitter (PBS) and sent to the DSF fibre. The resulting conjugate signals had wavelengths of 1553.0 nm and 1553.9 nm respectively.
Another example of using a phase conjugator with a plurality of channels in a WDM system is described in patent application WO 00/14917, in the name of Nokia Networks. According to the description, a first set of optical signals having different wavelengths from each other is transmitted from a first end of an optical fibre, and a second set of optical signals having different wavelengths from each other is transmitted from a second end of the same optical fibre. To optimize the costs of the optical network, the same wavelengths are used in the first and second sets, and a bidirectional phase conjugator is used in the optical fibre. The bidirectional phase conjugator changes each incoming wavelength into an outgoing wavelength by folding the incoming wavelength with respect to a folding wavelength. The signals received from both ends of the optical fibre have wavelengths identical to the wavelengths leaving the phase conjugator. In a preferred embodiment, the phase conjugator is located essentially in the centre of the optical path, in such a way as to compensate the chromatic dispersion. In another preferred embodiment, the terminal equipment uses a bidirectional optical amplifier having a non-uniform gain spectrum, and the channels are selected in such a way that the received wavelengths fall within the peak portion of the useful range of the gain curve and each of the transmitted wavelengths falls within the portion in which the gain curve is essentially flat.
The phase conjugation technique has also been studied for compensating third-order non-linear effects, such as FWM, self-phase modulation (SPM) and cross-phase modulation (XPM). For example, M. E. Marhic, N. Kagi, T.-K. Chang and L. G. Kazovsky, “Cancellation of third-order nonlinear effects in amplified fiber links by dispersion compensation, phase conjugation, and alternating dispersion”, Optics Letters, vol. 20, no. 8, pp. 863–5 (1995), show that it is theoretically possible to cancel the third-order non-linear effects in optical fibre paths. The necessary conditions are present in paths with two segments, with dispersion compensation, phase conjugation and amplification between the two segments, and also opposite chromatic dispersion coefficients in the two segments.
Paragraph 9.7.2 of the previously cited book by Agrawal also shows that chromatic dispersion and SPM can theoretically be compensated simultaneously by phase conjugation.
U.S. Pat. No. 6,175,435, held by Fujitsu Limited, describes an optical communication system, of the WDM type for example, which uses phase conjugation to compensate chromatic dispersion and the optical Kerr effect. A phase conjugator is placed between a transmission line I and a transmission line II. The desired compensation can be obtained if the value of the dispersion and the product of the non-linear refractive index and light intensity in each sub-section of the transmission lines I and II is specified in such a way as to be inversely proportional to the length of the section, and also if the corresponding ratio is equalized. In one embodiment, a plurality of additional optical fibres is used to provide additional compensation in case of WDM transmission. The WDM channels transmitted through an optical fibre SMF1 are converted by the phase conjugator and transmitted through an optical fibre SMF2 to be received by corresponding receivers. The signs of the dispersion upstream and downstream of the phase conjugator are equal. A frequency selection is carried out for each channel after the outgoing signal from the optical fibre SMF2 has been divided, and then a further compensation is carried out by using additional optical fibres, which are matched to the amount of residual compensation for the individual channels.
The use of compensators of chromatic dispersion or of the slope of the chromatic dispersion in conjunction with phase conjugators is described in other documents. For example, U.S. Pat. No. 5,532,868, held by AT&T Corp., describes an apparatus and a method for compensating the chromatic dispersion introduced by the signal conversion of an optical signal. In a described apparatus, a non-linear conversion means is placed within the path of an optical signal. The non-linear conversion means receives the optical signal and generates a converted optical signal. At least one dispersion compensator is placed within the signal path to provide a quantity of chromatic dispersion sufficient to counterbalance a portion of the chromatic dispersion introduced into the converted signal by the non-linear conversion means.
J. Yamawaku, M. Hanawa and M. Takahara, in “Timing jitter characteristics of the system using OPC's and DSCF's”, Technology and Infrastructure. NOC '98, pp. 126–31 (1998), propose a system configuration comprising two phase conjugators placed at L/4 and 3/4L (where L is the total length of the system), with a spacing of 60 km between the amplifiers. Each span between the amplifiers consists of a dispersion shifted fibre (DSF) with a length of 50 km and a dispersion slope compensation fibre (DSCF) with a length of 10 km. As shown in FIG. 2 of the article, the DSF and DSCF fibres have the same dispersion value at the signal wavelength. The phase conjugators compensate the chromatic dispersion and the Kerr effect, while the DSCF fibre compensates the third-order dispersion effects. The dispersion slope of the DSCF fibre is five times greater (in absolute value) than the slope of the dispersion of the DSF fibre. According to the authors, the use of a plurality of phase conjugators in the said system enables the spacing between the amplifiers to be increased without degradation of the transmission characteristics.
J. Piña, B. Abueva and C. Goedde, in “Periodically conjugated solitons in dispersion-managed optical fiber”, Optics Communications, pp. 397–407 (2000), present an analysis of long-distance propagation through short optical solitons (τ≈0.5 ps) in a non-linear optical fibre which incorporates the effects of periodic phase conjugation and dispersion management. In the treatment, carried out for a single channel, the effects of the frequency conversion introduced by the phase conjugators are disregarded.
To enable the chromatic dispersion to be exactly compensated by the MSSI technique, the fact that the chromatic dispersion undergone by the conjugated signal may be different from the dispersion undergone by the original signal has to be taken into account. This is because the wavelength of the phase conjugated signal is generally different in practice from the wavelength of the original signal, and, as noted above, the chromatic dispersion depends on the wavelength. For a single channel, perfect compensation of the chromatic dispersion by the MSSI method is possible if the phase conjugator is slightly shifted with respect to the exact centre of the optical path, in such a way as to allow for the different chromatic dispersion characteristics of the optical path downstream of the phase conjugator. This method is not adequate for a case where a plurality of channels is transmitted at different wavelengths.
U.S. Pat. No. 5,365,362, held by AT&T Bell Laboratories, describes an apparatus and a method for achieving bit rate-distance products of the order of 200 Tbit/s·km in non-soliton optical communication by using optical phase conjugation. The method and apparatus utilize phase conjugation in combination with an adjustment of the number of line amplifiers, their spacing, and/or their output power in order to compensate for the interaction between the first-order dispersion and the non-linearity dispersion effects in an optical fibre span. A description is also given of additional techniques for adjusting the system parameters, such as the dispersion-length products of the first and second portions of the fibre span, in order to compensate for changes in the first-order dispersion resulting from a non-zero second-order dispersion. As explained in U.S. Pat. No. 5,365,362, the proposed method and apparatus are also applicable to multi-channel systems, by using a multi-channel optical phase conjugator, as shown in FIG. 5 of the patent for example. The multi-channel phase conjugator comprises a first channel router, which receives the multi-channel optical signal on a common path and separates it into the different channels according to the wavelength or frequency of each channel. The multi-channel phase conjugator also includes a plurality of single-channel phase conjugators, each of which phase conjugates one of the channels by “four-photon mixing”. After the mixing has produced the phase conjugated output signal for a given input channel, the individual phase conjugated channels are recombined in a second channel router, in such a way that the desired phase conjugate of the whole multi-channel signal is obtained on the common output path. According to the inventors of U.S. Pat. No. 5,365,362, the frequency shift of each channel resulting from the phase conjugation should be such that optimal compensation of the second-order dispersion effects is possible for each channel. According to the description, one technique consists in carrying out the phase conjugation process in such a way that each phase conjugated channel undergoes the same minimum amount of dispersion. This can be achieved by phase conjugating each channel in such a way that its frequency after the phase conjugation is that of the adjacent channel to it before the phase conjugation.
According to the applicant, this technique for compensating the second-order dispersion effects is rather complicated and costly, since a multi-channel phase conjugator consisting of as many phase conjugators as there are channels in the system is a very complex device, particularly for systems with many channels. It is also disadvantageous when the number of channels in a system is increased, as a result of an increase in the required transmission capacity: in this case, a multi-channel conjugator of this type would have to be redesigned and/or replaced to allow for the introduction of the new channels.
The applicant has verified that, in a high transmission rate WDM system, it is possible that dispersion compensation using optical fibres with alternating dispersion, with a non-zero slope of the average dispersion with respect to the wavelength, will not permit satisfactory reception of a large part of the WDM signal, even if it is carried out with additional post-compensation optical fibres dedicated to each channel before the receiver. Because of the non-zero slope of the average dispersion, the channels with a higher average dispersion accumulate a large amount of dispersion. According to the applicant, these channels are more sensitive to distortion due to non-linear effects (such as XPM and SPM) which occur in the course of transmission along the optical line, so that a linear compensation of the dispersion before the receiver is no longer sufficient to recover an acceptable shape of the pulses. These non-linear effects can arise, in particular, for transmission at a high bit rate (>10 Gbit/s), as a result of the shorter duration of the transmitted pulses and the consequent higher peak power of the pulses.
The applicant has tackled the problem of compensating the chromatic dispersion over a broad band in a WDM system with a high transmission rate (>10 Gbit/s) in such a way as to obtain an acceptable level of reception for the channels included in this band, without the need to use optical fibres with special characteristics (such as inverted slope fibres) or fibre gratings with special characteristics.